While the detection of allergen-specific IgE, either by skin testing or in vitro assays, is a useful marker of allergic sensitivity, the relationship between specific IgE levels and allergic responses is far from clear. In fact, for food, drug, insect sting and inhalant allergens most studies have shown no relationship between the levels of specific IgE and type or severity of responses to allergen exposure. The lack of such a relationship may be due to a variety of factors that we propose to investigate in the clinical and ex vivo studies described in this project. A central hypothesis to be tested is that a combination of the allergen-specific IgE and total IgE levels, especially the ratio of these two measurements, by influencing FceRI occupancy and density, determines the threshold and likelihood for cellular and clinical reactivity. This hypothesis is based in part on what our group and other laboratories have learned about how IgE, and an FDA-approved IgE-lowering antibody therapy, omalizumab, regulates basophil and mast cell responsiveness and surface density of FceRI. Given the half-life of mast cells in different tissue compartments and the slower onset of action of omalizumab on mast cells compared to that for basophils, we propose to test the hypothesis that food-induced anaphylaxis and late phase responses are basophildependent responses by performing skin, airway and oral allergen challenge within the first two weeks, and after months, of omalizumab administration. We also hypothesize that mechanisms responsible for trafficking of eosinophils to sites of airway late phase responses involve IgE-mediated triggering of recruited basophils, which then release mediators capable of activating epithelial chemokine production selective for eosinophils. As part of these efforts, we propose to develop a new and reliable diagnostic test for the measurement of free total serum IgE levels in collaboration with colleagues in Core B. Our overall goal is to expand our knowledge of the biology of IgE and its clinical relevance using an approved anti-lgE antibody, omalizumab, as a mechanistic tool to modulate free IgE levels.